Abstract

Galaxies occupy different regions of the
OIII$łambda5007$/H$\beta$-versus-NII$łambda6584$/H$\alpha$ emission-line
ratio diagram in the distant and local Universe. We investigate the origin of
this intriguing result by modelling self-consistently, for the first time,
nebular emission from young stars, accreting black holes (BHs) and older,
post-asymptotic-giant-branch (post-AGB) stellar populations in galaxy formation
simulations in a full cosmological context. In post-processing, we couple
new-generation nebular-emission models with high-resolution, cosmological
zoom-in simulations of massive galaxies to explore which galaxy physical
properties drive the cosmic evolution of the optical-line ratios
OIII$łambda5007$/H$\beta$, NII$łambda6584$/H$\alpha$,
SII$łambdałambda6717,6731$/H$\alpha$ and OI$łambda6300$/H$\alpha$. The
line ratios of simulated galaxies agree well with observations of both
star-forming and active local SDSS galaxies. Towards higher redshifts, at fixed
galaxy stellar mass, the average OIII/H$\beta$ is predicted to increase and
NII/H$\alpha$, SII/H$\alpha$ and OI/H$\alpha$ to decrease -- widely
consistent with observations. At fixed stellar mass, we identify star formation
history, which controls nebular emission from young stars via the ionization
parameter, as the primary driver of the cosmic evolution of OIII/H$\beta$ and
NII/H$\alpha$. For SII/H$\alpha$ and OI/H$\alpha$, this applies only to
redshifts above $z=1.5$, the evolution at lower redshift being driven in
roughly equal parts by nebular emission from AGN and post-AGB stars. Instead,
changes in the hardness of ionizing radiation, ionized-gas density, the
prevalence of BH accretion relative to star formation and the dust-to-metal
mass ratio (affecting the gas-phase N/O ratio at fixed O/H) play at most a
minor role in the cosmic evolution of simulated galaxy line ratios.